Brakes for road vehicles, in particular large road vehicles such as lorries and coaches which have air brakes, are either drum brakes or disc brakes. Disc brakes provide generally better performance, because they are less prone to overheating which causes brake fade. Disc brakes also do not have the self-servo effect associated with drum brakes, thus giving the driver better control when braking. For these reasons, disc brakes have more stable performance.
In some markets, however, drum brakes are still preferred despite the accepted advantages of disc brakes. One reason for this is that drum brakes are mechanically simpler, and are generally easier to service without specialist equipment.
In a typical disc brake, there is an application unit within the disc brake caliper which provides a load train to transmit force from the actuating cylinder of the compressed air system to push the brake pad against the disc. The application unit typically includes a pair of tappets at the end of the load train, and a pivoting lever which is pushed by the actuating cylinder at one end, and which pushes against the tappets at the other end. In some designs, for example as described in WO2013143988, the pivoting lever acts indirectly on the tappets via a bridge, and in other designs, for example see EP1000263, the pivoting lever acts directly on the tappets. In both alternatives, the pivoting lever is usually of a “wishbone” shape, so that at the tappet end it can act on each tappet, or act on the bridge with two points of contact.
Part of the complexity which makes a disc brake more difficult to service comes from the way in which the brake adjusts to take up wear in the brake pad. Each tappet is made in two screw-threaded tappet sections, so that each tappet is adjustable in length by rotating one tappet section with respect the other tappet section. An adjuster is provided, the simplest mechanical type including a one-way sprag clutch and an over-torque clutch. The adjuster is actuated on brake application, if the running clearance between the disc and the pad is greater than a certain pre-set amount. The adjuster, when actuated, provides rotation. The over-torque clutch ensures that the brake is not over-adjusted when the brake pad, or caliper housing, elastically deforms during high-load applications, and the sprag clutch slips on the off-stroke in order that adjustment is not retracted. The tappets are rotatably linked, for example by a chain, timing belt or gear train, to the adjuster output shaft and to each other. In this way, the tappets rotate slightly when adjustment is required. This increases the length of the tappets slightly each time, so that the running clearance (the distance between the brake pad and the brake disc when the brake is not being applied) is maintained as the brake pad wears over time.
If the brake needs to be disassembled to replace a worn part, the adjuster usually needs to be taken out. In most current designs this cannot be done without interrupting the rotational link between the two tappets. If the rotational link is interrupted, the tappets may become set to different lengths, and if this is not corrected before reassembly the imbalanced application of the pad to the disc will result in potentially dangerous braking problems. Re-synchronizing the tappets is often a difficult procedure which requires specialist tools.
In some known designs, for example in WO2013079376, an adjuster is provided axially in-line with one of the tappets, and is connected to the tappet independently of the rotational linkage. Whilst this does mean that the adjuster can be removed without de-synchronizing the tappets, in practice it is not possible to remove an adjuster at all from this position with the application unit still installed within the caliper. Although a single unit including the two synchronized tappets can be separated from the adjuster, this is only possible by substantially disassembling the caliper and removing the unit.
In some older designs, for example in WO9701045, an adjuster is provided to the side of one of the tappets, between the tappet and the side of the caliper rather than between the tappets. In these designs, the adjuster can be removed without de-synchronising the tappets, but in order to do this a stressed rear cover plate has to be employed and this must be removed together with the operating shaft, which again amounts to a substantial disassembly of the caliper and removal from the vehicle.
More advanced adjusters than the simple mechanical type are becoming more popular. Electric adjusters which actively control the running clearance of the pad relative to the disc can provide for closer running clearance, which reduces the brake application time and therefore the stopping distance of the vehicle whilst ensuring that the brakes do not “run hot”, which reduces the efficiency of the vehicle and also the performance of the brakes. However, for the above mentioned reasons it is difficult to replace one type of adjuster with another type, because replacement of the adjuster requires substantial disassembly of the whole brake.
It is an object of the invention to provide a disc brake caliper which is easier to service.